A solid polymer fuel cell typically comprises an ion exchange membrane made of a solid polyelectrolyte membrane such as a perfluorosulphonic acid membrane; electrodes formed on both surfaces of the ion exchange membrane (a fuel electrode and an oxidizer electrode); and a charge collector. This is a device in which hydrogen is supplied to the fuel electrode, while the oxidizer electrode is supplied with oxygen or air, to generate electric power through an electro-chemical reaction. Each of the electrodes has a catalyst layer comprising a mixture of carbon particulates holding a catalyst material with a solid polyelectrolyte, and a gas diffusion layer (supply layer) made of a porous carbon material for supplying and diffusing a fuel and an oxidizing gas. The charge collector is made of an electrically conductive sheet made of carbon or metal.
In such a fuel cell, a fuel supplied to a fuel electrode reaches a catalyst through fine pores in a gas diffusion layer (supply layer). Then, the fuel is decomposed by the action of the catalyst to generate electrons and hydrogen ions. The electrons are led out to an external circuit through catalyst carriers (carbon particulates) and the gas diffusion layer (supply layer) within the fuel electrode, and flows into an oxidizer electrode through the external circuit. The hydrogen ions in turn reach an oxidizer electrode through an electrolyte in the fuel electrode and a solid polyelectrolyte membrane between both electrodes, and react with oxygen supplied to the oxidizer electrode and electrons flowing into the oxidizer electrode through the external circuit to produce water. As a result, electrons flow from the fuel electrode to the oxidizer electrode in the external circuit, electric power being taking out.
However, in a single solid polymer fuel cell unit basically configured as described above, the resulting voltage generated by the cell corresponds to a potential difference through oxidization and reduction at each of the electrodes, so that the fuel cell merely generates approximately 1.23 volts at maximum, even if it is an ideal open voltage. Thus, the fuel cell cannot always generate sufficient power as a driving power supply equipped in a variety of devices. For example, portable electronic devices typically require an input voltage of approximately 1.5–4 volts or higher as a power supply. For using the solid polymer fuel cell as a power supply for driving such a portable electronic device, unit cells of the fuel cell must be connected in series to increase the voltage generated thereby.
It is contemplated that unit cells are stacked for increasing the voltage to ensure a sufficient voltage. However, such a structure will be larger in overall thickness of the fuel cell, making this strategy unfavorable for a power supply for driving a portable electronic device which is required to be increasingly thinner.
Japanese Patent Laid-open No. 273696/96, for example, discloses a fuel cell assembly including a plurality of unit cells on the same plane, and a stacked structure which comprises a plurality of the fuel cell assemblies stacked one on another, as a technique for increasing a voltage generated by the cell while reducing the thickness of the cell.
Also, Japanese Patent Laid-open No. 171925/96 and Japanese Patent Laid-open No. 110215/2002 discloses a fuel cell assembly which has a single electrolyte membrane, a plurality of oxidizer electrodes on one surface of the electrolyte membrane, and a plurality of fuel electrodes on the other surface of the electrolyte membrane, such that a plurality of unit cells are disposed on the same plane.
Since the foregoing conventional fuel cell assembly is capable of generating a high voltage with a plurality of electrically connected cells, this fuel cell assembly provides a certain benefit in that a sufficient supply voltage can be obtained for driving an electronic device.
However, in the stacked structure described in Japanese Patent Laid-open No. 273696/96, the fuel electrodes and oxidizer electrodes of the respective unit cells disposed on a plane are not uniform in orientation, so that a fuel and an oxidizer gas must be supplied separately to each unit cell. Also, a retainer mechanism is required for sealing each unit cell in order to prevent the fuel and oxidizer gas within each unit cell from flowing into adjacent unit cells. These requirements make the spacing between the unit cells of the fuel cell assembly dependent on the dimensions of a mechanism for supplying the fuel and oxidizer gas as well as the retainer mechanism; therefore, it is difficult to achieve a sufficient reduction in size. In addition, the disclosed stacked structure requires a large number of components, and still has room for improvements in terms of a reduction in size and cost.
On the other hand, the fuel cell assembly disclosed in Japanese Patent Laid-open No. 171925/96 has a problem that hydrogen ions generated in a fuel electrode of a certain unit cell migrate (electrically leak) to an oxidizer electrode of an adjacent unit cell, not to an oxidizer electrode of the unit cell itself to cause a lower voltage. Particularly, the electric leak is remarkable when the unit cells are arranged at intervals as small as the thickness of the electrolyte membrane, inevitably reducing the voltage.
On the other hand, the throughhole connection system described in Japanese Patent Laid-open No. 110215/2002 has a problem of an electric leak caused by hydrogen ions migrating to an electrically conductive member inserted through a throughhole, in addition to an electric leak to adjacent oxidizer electrodes. Likewise, in this structure, the electric leak is more remarkable particularly when unit cells are arranged at smaller intervals, causing a larger reduction in voltage.